Generally, extraoral radiographic apparatuses have a similar structure: a rotating C-arm, supporting at one of its ends an X-ray source and at the other end an X-ray detector; the C-arm rotates about patient's head. The X-ray source generates an X-ray bundle, which is detected by the detector and processed by a computer, so allowing the reconstruction of two-dimensional (2D) or volumetric (3D) radiographic images. Said C-arm is supported by a column, which can vary its position in height in order to adjust it according to patient's stature.
In dentistry, different kinds of radiographic image are known: hereunder a brief description of different types of acquisitions is offered, to which reference will be made in the following.
Panoramic radiography (also known as orthopantomography, abbreviated as PAN) produces a radiographic image of a curved plan approximating patient jaws, with blurring of the anatomical structures laying outside a narrow layer around the predesigned curved plane. It is a planar radiography, to which reference will be made in the following as 2D, too.
Cone beam volumetric radiography (also known as CBCT or 3D) is the acquisition, from different projection angles, of a series of two-dimensional radiographic images, which will be processed column-acquisition to reconstruct three-dimensional volumes.
Teleradiography (abbreviated as CEPH) is a projective radiographic technique, producing radiographic images of the skull or of other anatomical areas from different projections, with minimum magnification and geometrical distortion. Usually two perspectives are represented, latero-lateral and antero-columnerior.
On the market hybrid apparatuses are available, which acquire both panoramic images (2D) and volumetric images (3D)
It is worth noting that teleradiographies, also called cephalometries, can be obtained in two ways:
A direct way, making use of a teleradiographic or hybrid apparatus having a CEPH arm capable of setting patient's head and X-ray sensor at a suitable distance from X-ray source (distance source-detector of about 165 cm);
An indirect way, making use of a CBCT apparatus. In this case, the acquisition is performed positioning the patient at the habitual distance for a CBCT apparatus (i.e. CEPH arm is not necessary). To use a CBCT apparatus to acquire teleradiographic images, first a normal CBCT acquisition of patient's head has to be performed; then the raw images acquired by the sensor are processed using a suitable algorithm (e.g. Feldkamp) so as to obtain a 3D reconstruction of the volume of interest. The volumetric data are then used to obtain an orthogonal or slightly perspectival projection of patient's head, using the absorption values of the different voxels of the reconstructed volume to recreate the necessary transparencies to obtain a teleradiographic image;
According to the orientation of the projection with respect to patient's head, an antero-columnerior (AP) teleradiography or a latero-lateral (LL) teleradiography can be obtained.
For diagnostic reasons CEPH images have to be acquired using patient positioning devices without bite and without chin rest; furthermore in this case, as the volume to be acquired is much bigger than dental arches only, the patient positioning device should also be radio-transparent.
Teleradiographies allow clinical staff to prepare cephalometric tracings of the skull.
Although in any kind of radiographic acquisition keeping patient's head still is fundamental in order to prevent movement artefacts, positioning devices vary because the requirements connected to the positioning itself vary. In particular, when performing a panoramic or a CBCT acquisition, e.g. for orthodontic of implant applications, customarily the patient keeps a bite between her/his incisors, so that they are not overlapping, and radiographs can be used for diagnosis. On the other hand, when performing teleradiographies, the position of incisors, and in general occlusion, must be natural and therefore cephalostats are not provided with a bite and a chin rest.
To obtain a good diagnosis, the head should have ear canals and the intersection of the frontal bone and two nasal bones, called nasion, aligned through radio-opaque markers, which are part of the cephalostat, so as to highlight anatomic anomalies or incorrect positioning of the patient.
In particular, nasion is defined as “The craniometric point at the bridge of the nose where the frontal and nasal bones of the skull meet.”
Cephalostats, i.e. the devices for positioning and alignment, in most cases comprise a moving system of two radio-transparent, lateral shafts supporting earpieces and markers, and a front shaft leaning against patient's nasion.
Commonly cephalostats having the moving mechanism (opening and closing of shaft and nasion) supported from top are used, but anyway the cephalostat lays outside of the X-ray bundle, so that the X-ray bundle does not impinge on metallic or non-radio-transparent components, preventing the generation of metal artefacts. Such movement is generally manual, and the cephalostat is fixed permanently and integrally to the apparatus; it allows adjusting the patient positioning device to different skull dimensions.
In CBCT tomographs and for conventional orthopantomographs even radio-transparent craniostats for positioning patients are used, which lack the aligning function; they indicatively comprise a chin support, a forehead support, temples support and a bite.
EP0534548B1 of Dentsply describes a cephalostat for extraoral radiographic apparatuses having a support for nasion, which is movable along a vertical axis and pivoting around the same axis, so that patient's head can be brought in the desired position for acquiring radiographic images.
It is worth specifying that known art cephalostats are radio-transparent in their portions in contact with the patient, but have a portion on which X-rays do not impinge, allowing the movement of the portions in contact with the patient, which is generally made of metal and therefore not radio-transparent, and capable of generating metal artefacts.
WO2007134213 to Xoran Technologies describes the possibility of using a CBCT acquisition to identify the points necessary to perform a cephalometric acquisition.
EP534548 discloses a cephalostat which is provided with adjustable ear plugs and a frontal support enabling immobilizing of an upper jaw of a patient without immobilizing its mandible. The ear plugs guarantee a well defined positioning of the patient's jaw with respect to the beam path of an imaging X-ray beam. The frontal support has preferably the shape of a nose support. The cephalostat is interchangeable with a bite or a chin rest on different Xray imaging apparatus.